Among recent projectors, some projectors are provided with LEDs of three colors, red, green and blue. They use display elements such as liquid crystal panels to spatially modulate light from the LEDs for each color to generate image light of each color, and that then project a color image made up by this image light.
Typically, the output characteristics differ for each of the red LED, green LED, and blue LED, the maximum output value of the red LED and green LED being smaller than that of the blue LED.
When each of the red LED, green LED, and blue LED is driven at maximum output, the luminance of the projected image also reaches a maximum. However, the color mixture ratio of the light (red, green, and blue) from the LEDs for each color that are being driven at maximum output differs from the predetermined color mixture ratio for obtaining the ideal white balance, and the projected image therefore has unnatural colors and deteriorates picture quality. As a result, the ideal white balance is normally obtained by controlling the output of the blue LED while using the maximum output value of the red LED or green LED as a reference.
The optical output of the light source itself can be increased by arranging red or green LEDs in an array. A projector, however, is subject to the limitation known as etendue that is determined by the area of the light source and the angle of divergence, and if the value of the product of the area of the light source and the angle of divergence is not made equal to or less than the product of the area of the display element and the acceptance angle (solid angle) that is determined by the f-number of the projection lens, a portion of the light from the light source is not used as projection light. Accordingly, despite the arrangement of a large number of LEDs in array form, the brightness of the projector cannot be improved beyond the limitations of etendue.
In order to achieve higher luminance in the above-described projector, the output of the red LEDs and green LEDs must be increased without raising etendue, and to achieve this goal, increasing the luminance of these light sources is crucial.
The luminance of LEDs can be increased by increasing the drive current. However, an increase in the drive current is attended by the generation of heat in the LEDs themselves, and this heat detracts from the efficiency of light emission of the LEDs. As a result, when the drive current surpasses a particular value, the optical output is saturated, and the output does not increase despite increases in the drive current. This type of “thermal saturation” limits the increase in luminance of LEDs realized by increases in the drive current.
As described hereinabove, the achievement of higher luminance of a projector raises the two problems of etendue and the influence of heat.
Patent Document 1 discloses a light source device as a technology that can solve these two problems and achieve higher luminance.
The light source device described in Patent Document 1 is used in a projector and includes: a plurality of solid-state light-emitting devices that are arranged in a ring such that the light emitted from each solid-state light-emitting device converges at a single point, light guide means that reflects the emitted light from each of the solid-state light-emitting devices in the same direction and guides this reflected light along the same light path, and control means that causes each solid-state light-emitting device to sequentially and intermittently generate light.
The light guide means has a reflection surface and rotation means that causes the reflection surface to rotate such that emitted light from each solid-state light-emitting device is reflected in the same direction.
The control means causes each solid-state light-emitting device to sequentially generate light in synchronization with the rotational motion of the reflection surface realized by the rotation means.
By means of the above-described light source device, light that is emitted from each solid-state light-emitting device and reflected by the reflection surface is guided along the same light path, whereby etendue does not increase.
By causing each solid-state light-emitting device to sequentially and intermittently generate light, the reduction of the efficiency of light generation caused by the generated heat of each solid-state light source can be suppressed, whereby the output of the light source device can be increased. A brief explanation for these effects is next presented.
The maximum amount of input current (the amount of current at which thermal saturation is produced) under pulsed operation typically greater than that under continuous operation. This is because the repetition of operation and non-operation at a fixed interval in a pulse operation results in a lower time-averaged current value than the current during operation, and the effect of generated heat is correspondingly lower.
According to the light source device described in Patent Document 1, each solid-state light-emitting device is operated in pulsed mode, whereby the maximum amount of input current of each solid-state light-emitting device is greater than that in continuous mode. As a result, a greater current can be supplied to each solid-state light-emitting device than in a case in which one solid-state light-emitting device is lighted continuously, and the luminance of each solid-state light-emitting device can be increased.
Each solid-state light-emitting device is operated sequentially and intermittently, whereby the output light of the light source device can be considered continuous light, and this light output is greater than for a case in which one solid-state light-emitting device is operated continuously. Accordingly, this light source device increases the light output without increasing etendue and can realize higher luminance.
A projector that has greater luminance can be provided by applying the light source device described in Patent Document 1 to the light source of each of the colors red, green, and blue of a projector. In addition, although the red and green LEDs have less output than a blue LED as described hereinabove, balance can be established by adjusting the number of LEDs used in the light source device of each color.
In Patent Document 2, a display device is described in which two green solid-state light sources are provided to increase green light.
The display device described in Patent Document 2 includes: first to third illumination units, a first display panel into which light from the first illumination unit is irradiated, a second display panel into which light from the second illumination unit is irradiated, a third display panel into which light from the third illumination unit is irradiated, an optical element into which image light from the first to third display panels is entered from respectively differing surfaces and in which image light that was entered from each surface is superposed and exited, and projection lens that projects image light from the optical element onto a screen.
The first illumination unit is equipped with a red solid-state light source, a first green solid-state light source, and a dichroic prism that transmits red light from the red solid-state light source and reflects first green light from the first green solid-state light source. In this first illumination unit, the red light from the red solid-state light source and the first green light from the first green solid-state light source are incident to the first display panel by the same light path by way of the dichroic prism.
The second illumination unit is equipped with a second green solid-state light source. The second green light from the second green solid-state light source is irradiated to the second display panel.
The third illumination unit is equipped with a blue solid-state light source. The blue light from the blue solid-state light source is irradiated to the third display panel.
In the display device described in Patent Document 2, the red solid-state light source and the first green solid-state light source are lighted in time division, and an image based on the video signal for red light is displayed on the first display panel during the lighted period of the red solid-state light source, and an image based on the video signal for green light is displayed on the first display panel during the lighted period of the first green solid-state light source.
The second green solid-state light source and the blue solid-state light source are lighted constantly. An image based on the video signal for green light is displayed on the second display panel and an image based on the video signal for blue light is displayed on the third display panel.
According to the display device according to Patent Document 2, while the first green solid-state light source is lighted, first green light from this first green solid-state light source is added to the second green light from the second green solid-state light source, whereby the luminance of the green image is increased. The use of this method improves the luminance of the color for which the maximum light source output is small, whereby the overall brightness of the projector can be improved.